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should be longer (i.e., require a slightly longer cycle length or longer splits for approaches
that peak heavily) than otherwise indicated by the total peak hour volume.
Key Point: The peak hour factor is a measure of how much volumes vary
within the peak hour. When the PHF is less than 0.85, you should account
for volume variations within the peak period when you are computing your
signal timings.
Average Daily Traffic
ADT information is a valuable resource for checking the accuracy of peak hour TMCs,
checking for the location of the peak hour for each intersection approach, and monitoring
increases in overall traffic volume over time. The figure below shows a hypothetical
intersection and hourly counts that develop the one-way ADT counts for each intersection
approach roadway.
Time
12-1am
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11
11-12
12-1pm
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11
11-12
TOTAL
SB ADT
WB ADT
EB ADT
NB ADT
NB ADT
0
0
0
0
0
50
100
200
200
100
50
50
100
50
50
100
200
350
200
50
50
50
0
0
1950
SB ADT
0
0
0
0
0
50
100
200
200
100
50
50
100
50
50
100
200
350
200
50
50
50
0
0
1950
EB ADT
0
0
0
0
0
50
100
200
200
100
50
50
100
50
50
100
200
350
200
50
50
50
0
0
1950
WB ADT
0
0
0
0
0
50
100
200
200
100
50
50
100
50
50
100
200
350
200
50
50
50
0
0
1950
Examination of the hourly figures used to calculate ADT confirms that the 350 vehicles
counted from each direction are roughly the same as the TMC numbers shown for about
that same time period (5:00 to 6:00 PM). This check helps validate that neither the TMC
values nor the ADT values are out of scale with representative values. The high count for
the hour from 5:00 to 6:00 PM also shows that the hourly volume used to compute ADT
numbers indicate what we already discovered – that the PM peak is somewhere around the
5:00 to 6:00 PM range, and it turns out from the TMC that the actual peak is from 4:45 to
5:45 PM.
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